1. Field of the Invention
This invention relates to an apparatus for coating flexible, flat sheets with a thin layer of curable liquid material.
2. Description of the Related Art
Coatings are used for a variety of applications. For example, relatively thin transparent coatings are often applied to sheet stock in order to protect the underlying layer from abrasive damage. Clear protective coatings applied to photographic products protect the underlying image during repeated handling.
One particularly useful application for a protective coating is its use in the printed circuit industry on phototools (also referred to as photographic masks or stencils). Phototools have an intricate, complex image that represents an electrical circuit often containing many fine lines and junctions spaced closely together. In use, a circuit print is made by exposing a photoresist laminate to high intensity light shining through the phototool. In this manner, a single phototool sheet can be used repeatedly to make a number of prints.
It is important that the photographic image prepared on the phototool remains free of defects since defects on the phototool will be reproduced on the photoresist laminate and thus on the printed circuit. Many printed circuit manufacturers inspect the image on the phototool with a microscope or with an automatic optical scanner on a frequent basis to insure that the image lines are continuous and not defective. Depending on the size and intricacy of the image, microscopic inspections can take up to two or three hours.
Repeated handling of phototools during the manufacture of printed circuits, however, often leads to scratches on the emulsion of the phototool. Abrasions can result from many sources: particles of dirt, grit, small pieces of metal or sharp burrs (the latter are often found on the edges and corners of copper laminated sheets that serve as a backing for the printed circuit and carry the photoresist layer). Consequently, it is often desired to apply a clear protective coating to phototools to help avoid damage to the image. U.S. Pat. Nos. 4,623,676 and 4,668,601 (both assigned to the assignee of the present invention) describe radiation-curable compositions which are particularly satisfactory when used as a protective coating for phototools. Other abrasion-resistant compositions are described in U.S. Pat. Nos. 4,156,046, 4,293,606, 4,497,861, 4,619,949, all of which are assigned to the assignee of the present invention.
In the past, the coatings described in U.S. Pat. Nos. 4,623,676 and 4,668,601 have been applied to one side of flexible sheets by coating each sheet by hand or by passing the sheet through a nip between a rubber-coated drive roll and a wire wound applicator roll that is spaced a slight distance from a steel fountain roll. The coated sheet was transferred by hand (or by gravity along an inclined chute) to a flat table or conveyer belt, and a source of radiation was activated to cure the coating. However, flexible sheets tend to curl during the curing operation due to shrinkage of the coating composition that is present on only one side of the sheets.
There has been a need in the art for an automated apparatus that handles thin flexible sheets during a coating and curing operation. However, one problem in this regard is the desire to avoid any contact with the coated side of the sheet before the coating is cured so that the entire area of the sheet can be coated without defects. While a hold-down clamp or the like could be employed along the edge portions of the sheet during the curing operation, the coating, if any, on the sheet beneath the clamps would likely be distorted and/or not completely cured. Also, trimming the edge portions and cleaning the coating material from the clamps after each coating operation would be completely unsatisfactory in many applications and also an inconvenience.